The present invention relates to a voltage non-linear resistor and method of fabricating voltage non-linear resistor made of ZnO as the main component mainly used in a electric power field such as a transmission/transforming system.
Since the voltage non-linear resistor made of ZnO as the major constituent (hereinafter referred to as a "ZnO element") has excellent non-linear current/voltage characteristics, it has been widely used as an arrester element in a transmission/transforming system. The voltage non-linear resistor is formed of the main component of ZnO containing Bi oxide as a main additive and small amounts of oxides of Sb, Mn, Co, Cr, Si, Ni, Al, B as sub-additives through a common ceramic fabrication technology. The common ceramic fabricating technology here means processes of mixing, calcining and granulating of raw material powder, compacting the powder to form the powder in a proper shape such as a disk, plate, cylinder or torus, baking and heat-treating the compacted body to form a sintered body, then forming electrodes.
The voltage non-linear resistor for electric power use fabricated through the above precesses is required to have various important characteristics such as a high non-linear coefficient (.alpha.-value), optimization of limiting voltage (varistor voltage), increase of impulse withstanding ability, improvement of loading life time and so on. The most important characteristic among them is that current does not short-circuit to flow along the side surface of the ZnO element when an impulsive high voltage such as a thunder surge, switching surge or the like is applied to the ZnO element (prevention of creeping short-circuit).
In order to cope with this requirement, there are proposed some methods for preventing the creeping short-circuit current flow along the surface of the ZnO element by forming an inorganic high resistance layer having a resistivity higher than that of the ZnO element itself on the side surface of the ZnO element through applying and bake-attaching processes. Typical examples of the inorganic high resistance layers are made of boron silicate zinc glass and aluminum silicate glass as disclosed in Japanese Patent Publication No. 54-26710 (1979) and Japanese Patent Publication No. 58-27643 (1983).
Prevention of creepage short-circuit of a ZnO element maintain the stability of an arrester using the ZnO element, which leads to improvement of reliability and safety of the transmission/transforming system itself.
The voltage non-linear resistors of the prior art described above have the following disadvantages from the viewpoint of prevention of the creepage short-circuit. In a case of forming a boron silicate zinc glass layer, the non-linear coefficient for the ZnO element is decreased. Further, since the acid-resistivity of the glass is low, there is a disadvantage in that the creepage short-circuit resistivity is decreased due to corrosion of the glass by nitric acid gas produced by corona discharge when the ZnO element is used by being contained in a nitrogen atmosphere as in an arrester. Furthermore, in a case of aluminum silicate glass which is proposed to eliminate the above disadvantages, according to the inventors' experiment result using the glass having the same chemical composition and the same component ratios as disclosed, wetness between the ZnO element and the glass itself is worse. Thereby, there is a problem of decrease in the creepage short-circuit resistivity because micro-cracks occur from the interface between the element and the glass layer during the fabricating process and during use as an arrester to cause separation of the glass layer as a result.